1. Field of the Invention
The invention relates to a cooling system and method of a fuel cell which generates electricity by an electrochemical reaction produced by supplying a fuel gas to an anode side and supplying an oxidizing gas to a cathode side.
2. Description of the Related Art
Fuel cells are being used in vehicles because they have little effect on the environment. A fuel cell produces the necessary power by, for example, supplying a fuel gas such as hydrogen to an anode side of a fuel cell stack and an oxidizing gas that includes oxygen, such as air, to a cathode side, and producing a reaction between the two through an electrolyte membrane. This reaction generates heat in the fuel cell so to cool it, a coolant such as cooling water is circulated through the fuel cell stack and then cooled by a radiator or the like. In order to warm up a cold fuel cell at startup, the coolant is heated to an appropriate temperature by a heater, for example. In this way, the coolant is circulated through the fuel cell stack and its temperature adjusted.
Also, a gas compressor such as an air compressor (ACP) is used to appropriately pressurize the oxidizing gas supplied to the cathode side of the fuel cell stack. As the ACP operates, it also generates heat and is therefore cooled by a heat exchanger, referred to as an intercooler. In addition, vehicles are also provided with a heat exchanger for air-conditioning the cabin. In this way, vehicles are provided with various heat exchangers for different purposes so it would be reasonable to consider their shared use.
For example, Japanese Patent Application Publication No. JP-A-2005-79007 describes a fuel cell system that prevents clogging while helping to make up for insufficient humidity from a humidifier provided on the cathode side of the fuel cell. Here, both coolant for cooling the cathode supply gas and coolant for cooling the fuel cell stack are shared and the heat exchanger for cooling the cathode supply gas and the fuel cell stack are connected in series by a coolant flow passage. Two three-way valves which change the direction of coolant flow depending on whether the temperature of the fuel cell is high or low are arranged midway in this coolant flow passage.
Also, Published Japanese National Phase Application No. 2005-514261 of PCT application describes a method for heating and cooling a vehicle having a fuel cell as an air-conditioning apparatus and heating source which easily compensates for insufficient heat when the external temperature is low, and which also enables both sufficient cooling of the fuel cell apparatus and comfortable air conditioning when the external temperature is high. According to the technology described in that publication, a common coolant is used to cool the heating source and provide air conditioning for the vehicle so only a single coolant circuit is used. This coolant circuit branches off into a first sub-circuit and a second sub-circuit at a branching point. The first sub-circuit distributes coolant to the fuel cell apparatus, while the second sub-circuit distributes coolant to the air conditioning apparatus of the vehicle. After circulating through these apparatuses, the distributed coolants then merge together again at a merging point. That is, a heat exchanger of the fuel cell apparatus and a heat exchanger of the air conditioning apparatus are arranged in series in a single loop.
In the related art, when independently controlling these heat exchangers, the coolant circuit and the control thereof are independent from one another which is inconvenient. When the cooling system of the fuel cell stack and the cooling system of the cathode supply gas are controlled independently, the temperature of the cathode supply gas entering the fuel cell stack is determined by the cooling system of the cathode supply gas, and the temperature of the cathode supply gas (so-called cathode off gas) exiting the fuel cell stack is mostly determined by the cooling system of the fuel cell stack. If these two cooling systems are controlled independently from one another, the temperature difference between temperature of the cathode supply gas entering the fuel cell stack and the temperature of the cathode off gas may become too large, which may result in the following problems.
For example, a humidifier is provided parallel with the fuel cell stack to appropriately humidify the cathode supply gas and supply that humidified gas to the fuel cell stack, but the temperature difference between both ends of the humidifier may become too great. The humidifier used may have a well-known tubular construction, but if the temperature difference between both ends of the humidifier becomes too great, this tubular construction may become damaged and not work sufficiently. Thus, having the coolant circuits and controls thereof independent from one another for each of the fuel cell heat exchangers not only makes the structure complicated, but also results in inefficient use of the coolant and may lead to problems such as that described above.
Japanese Patent Application Publication No. JP-A-2005-79007 and Published Japanese National Phase Application No. 2005-514261 of PCT application describe related art which share the coolant for cooling the cathode supply gas and the coolant for cooling the fuel cell stack and use a common coolant for cooling the fuel cell, i.e., the heating source, and providing air conditioning for the cabin of the vehicle. With these technologies, a fuel cell stack and another heat exchanger for cooling are arranged in series in the coolant flow passage and the same coolant is shared. As a result, regulation of the temperature of the fuel cell stack and regulation of the temperatures of the cathode supply gas and the vehicle cabin are interdependent. Accordingly, although the coolant is used more efficiently in these technologies, the respective temperatures can not be controlled independently. Thus, it is just as difficult to appropriately regulate the temperatures as it is when the respective cooling systems are controlled independently.
In this way, in the related art, temperature regulation of the fuel cell stack and temperature regulation of the cathode supply gas and the vehicle cabin are not cooperatively controlled.